Laminates for use in flexible printed wiring boards and HDD suspensions have been produced so far by pasting a polyimide or polyester film on a conductor by the use of an adhesive. However, the laminates produced in this manner presented problems such as lowered heat resistance and fire retardance because of the presence of an adhesive layer. Moreover, they suffered a large dimensional change when the conductor was etched or some sort of heat treatment was applied to them and this dimensional change was an obstacle in later manufacturing steps.
To solve the aforementioned problems, a process is disclosed in JP6-93537B for providing a base material for a flexible printed wiring board which is highly reliable in respect to dimensional stability against temperature change, adhesive strength and post-etching planarity by coating a conductor directly with polyimide and forming an insulator composed of plural layers of polyimides differing one another in the coefficient of thermal expansion. However, this patent takes no interest in the coefficient of hygroscopic expansion of polyimide in use and, in the cases where the polyimide layers in the base material exhibit a high coefficient of hygroscopic expansion, there arose a problem of the base material suffering a loss in dimensional stability by a change in ambient moisture, even though unaffected by a thermal change. That is, a polyimide layer exhibiting a high coefficient of hygroscopic expansion not only causes such troublesome phenomena as warpage, curling and twisting of the laminate as the ambient humidity changes but also degrades the electrical properties of the laminate after fabrication of a circuitry.
Therefore, there is a demand for development of laminates which change little dimensionally as the ambient humidity changes or laminates exhibiting a low coefficient of hygroscopic expansion.
An object of this invention is to provide a humidity-resistant laminate comprising a polyimide layer formed by coating and exhibiting a low coefficient of hygroscopic expansion and to provide a process for producing the same. Another object of this invention is to provide a humidity-resistant laminate comprising a resilient conductor layer required for maintaining a certain minute gap in such a device as an HDD suspension.